We investigated the transmigrating activity of peripheral blood T cells of patients with human T-lymphotropic virus type I-associated myelopathy (HAM) through reconstituted basement membrane. The transmigrating activity of CD4+ T cells of HAM patients was increased significantly in comparison to that of anti-HTLV-I-seropositive carriers and HTLV-I-seronegative controls. However, the migrating activity of CD8+ T cells was not significantly different in HAM patients and controls. The activity of aminopeptidase-N in peripheral blood T cells of HAM patients also was increased significantly, as compared to that of controls. In addition, HTLV-I proviral load in transmigrating CD4+ T cells of HAM patients was increased significantly (two- to eight-fold), compared to that in nontransmigrating CD4+ cells. By contrast, no significant difference in HTLV-I proviral load was found between transmigrating and nontransmigrating CD4+ cells of HTLV-I-seropositive carriers, although copy numbers of HTLV-I proviral load were very low in them. The heightened transmigrating activity of CD4+ cells from HAM patients through reconstituted basement membrane is based on the increased activity of aminopeptidase-N. Collectively, these findings suggest that HTLV-I-infected CD4+ T cells play an important role in the early stage of the pathogenesis of HAM.